1. Field of the Invention
The invention relates to an x-ray examination apparatus comprising a frame to which are connected an x-ray source for emission of an x-ray beam and an x-ray detector that is placed opposite the x-ray source, and
filter means that are placed between the x-ray source and the x-ray detector, the filter means comprising an x-ray absorbing filter body and drive means for positioning the filter body in the x-ray beam. PA1 two holding members having a central opening positioned around a central ray connecting the x-ray source and the x-ray detector, the filter body being placed between the holding members and being movably connected thereto via a guiding means, PA1 a motor coupled to at least one holding member for rotation of the holding member around the central ray and PA1 coupling means for coupling and uncoupling the holding members joint rotation of the holding members around the central ray causing the filter body to be fixed relative to the holding members, rotation of one of the holding members with respect to the other holding member causing the filter body to move relative to the holding members via the guiding means.
The invention also relates to a filter means for use in such an x-ray examination apparatus.
2. Description of the Prior Art
An x-ray examination apparatus of the above-mentioned kind is known from the European Patent EP-B1-157 688 which corresponds to U.S. Pat. No. 4,670,896.
From the above-mentioned patent it is known that when objects having a mutually different x-ray absorption appear in the same x-ray image, such as heart and lungs, the x-ray image shows very bright areas next to very dark areas. Structures in the x-ray image having only a small difference in contrast falling within these areas are difficult to observe. By placing a wedge-shaped filter body in the x-ray beam such that the x-ray beam is attenuated in the areas of high transmissivity, the average contrast in the picture is reduced, such that the dynamic range of the x-ray image can be adjusted to cover the range of absorption values of the details that are of interest. Thereto first an x-ray image is made that is optically projected onto the patient. By placing wedge-shaped filter bodies, consisting for instance of plastic and lead, having an optical absorption corresponding with the x-ray absorption of the filter body in the light path, a visual indication of the filter position is obtained. After the filter bodies are in the desired position the final x-ray picture is taken. In an x-ray examination apparatus of the above-mentioned kind, the x-ray beam is collimated by two pairs of x-ray absorbing shutters the projection of which onto the patient determines the field of view. These shutters are movably mounted within a housing that is close to the x-ray source. In the same housing a mirror is present to project light onto the patient from a position corresponding to the position of the x-ray source such as to obtain an optical indication of the field of view. An extra filter that can be placed into the x-ray beam is preferably mounted in the same housing with the mirror and the collimating lead shutters and should therefor take up little space.